1. Field of the Invention
The present invention relates to an optical fiber and an optical fiber preform.
2. Description of the Related Art
An optical fiber composed of silica-based glass and having a core region doped with an alkali metal is known. In the case where a core portion of an optical fiber preform is doped with an alkali metal, it is possible to reduce the viscosity of the core portion during drawing of the optical fiber preform, and relaxation of a network structure of silica glass can be made to enhance. It is believed that, consequently, an attenuation of the resulting optical fiber can be decreased.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-537210 and U.S. Patent Application Publication No. 2006/0130530 disclose a diffusion method as a method of doping silica glass with an alkali metal. In the diffusion method, a glass pipe is heated by an external heat source or plasma is generated in the glass pipe while vapor of a raw material such as an alkali metal element or an alkali metal salt is introduced into the glass pipe, thereby doping an inner surface of the glass pipe with an alkali metal by thermal diffusion.
After the doping into the inner surface of the glass pipe with the alkali metal in this manner, the glass pipe is heated to be shrunk to a certain diameter. After the glass pipe has been shrunk, in order to remove contaminated impurities, such as Ni, Fe, and other transition metals, which were also doped in the glass pipe at the same time as the doping of the alkali metal, the inner surface of the glass pipe is etched by a certain amount in the direction of the wall thickness. The diffusion rates of alkali metals are higher than those of transition metals. Therefore, even when the transition metals are almost perfectly removed by etching the inner surface of the glass pipe by a certain amount in the thickness direction thereof, considerable amount of the doped alkali metal remains in the glass. After the etching, the glass pipe is made to collapse by heating to manufacture an alkali-metal-doped core rod. A cladding portion is synthesized on the outside of the alkali-metal-doped core rod, thus manufacturing an optical fiber preform. Subsequently, the optical fiber preform is drawn to manufacture an optical fiber.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) Nos. 2007-504080, 2008-536190, and 2010-501894 describe that a core region of an optical fiber is substantially composed of pure silica glass, that the chlorine concentration is preferably low in an optical fiber having a core region doped with an alkali metal, and that a pure-silica-core fiber having a germanium-free core region preferably contains substantially no chlorine and the chlorine concentration of the fiber is preferably 500 ppm by weight or less.
Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2009-541796 discloses an optical fiber having a core region containing an alkali metal in a concentration of 50 to 500 ppm and chlorine in an average concentration of 500 ppm by weight or more. However, as is apparent from the description of paragraph [0029] and FIG. 2, the chlorine concentration in a first core region that is mainly doped with an alkali metal is as low as 100 ppm by weight or less.